1. Field of the Invention
The present invention relates to zoom lens systems.
2. Description of the Related Art
A zoom lens having a high zoom ratio and a high optical performance in a whole zooming region is being demanded for a photographing optical system in an image pickup apparatus, such as a video camera or a digital still camera using a solid-state image pickup element, or a silver-halide type camera using a silver-halide film.
A zoom lens meeting the demand may be a four unit zoom lens including four lens units of a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, and a fourth lens unit having a positive refractive power, in that order from an object side to an image side.
In the four unit zoom lens, the second lens unit moves for magnification variation, and the fourth lens unit compensates an image plane variation caused by the magnification variation and performs focusing. (For example, see (1) Japanese Patent Laid-Open No. 08-304700, (2) Japanese Patent Laid-Open No. 2000-121941, (3) Japanese Patent Laid-Open No. 2003-295053, which corresponds to U.S. Patent Application Publication No. 2003/0189763, (4) Japanese Patent Laid-Open No. 05-060971, which corresponds to U.S. Pat. No. 5,638,216, (5) Japanese Patent Laid-Open No. 2005-242014, which corresponds to U.S. Patent Application Publication No. 2005/0007480 and No. 2005/0190458, (6) Japanese Patent Laid-Open No. 07-128619, (7) Japanese Patent Laid-Open No. 11-237550, which corresponds to U.S. Patent Application Publication No. 2002/0024732 and No. 2002/0063961, (8) Japanese Patent Laid-Open No. 11-344669, which corresponds to U.S. Patent Application Publication No. 2005/0195482 and No. 2005/0270646, (9) Japanese Patent Laid-Open No. 2002-244037, and (10) Japanese Patent Laid-Open No. 2003-322795.)
In a typical zoom lens, if the refractive power of each lens unit is increased, a moving distance of each lens for obtaining a certain zoom ratio unit is decreased. Thus, the total lens length can be decreased while the certain zoom ratio can be provided, thereby easily downsizing the entire system.
However, merely increasing the refractive power of each lens unit to decrease the moving distance of each lens unit may increase an aberration variation during zooming. It is difficult to obtain a high optical performance in the whole zooming region.
Particularly in a rear focus four unit zoom lens, it is important to properly set the lens arrangements of the first and third lens units provided on the object side and the image side of the second lens unit for magnification variation, so as to obtain a high optical performance in a whole screen.